It is known that electromagnetic communication systems employ broad bandwidth techniques, such as the so-called frequency-agile or frequency-hopping systems in which both the transmitter and receiver rapidly and frequently change communication frequencies within a broad frequency spectrum in a manner known to both units. When operating with such systems, antennas having multiple matching and/or tuning circuits must be switched, whether manually or electronically, with the instantaneous frequency used for communications. As such, there is a need for a single antenna reasonably matched and tuned to all frequencies throughout the broad frequency spectrum of interest.
In a particular frequency range of interest—500–2500 MHz—a short “stubby” dipole antenna has been thought to be a promising antenna construction. These short stubby cylindrical dipoles provide a low length to width ratio for obtaining wide operational bandwidths. Unfortunately, these constructions suffer at the higher operational end of their useful band with natural current nulls and current reversals. This effect is a natural phenomenon of diminishing wavelength with increasing frequency. As a result, the antenna becomes too long for the desired end use. And the reversal currents start to move toward the center of the antenna element as the operating frequency is increased. Additionally, the elevation pattern is adversely effected. When this happens a null or pattern depression is created at 0° elevation. An even further increase in frequency results in an elevation pattern bifurcation.
These undesirable characteristics are evidenced in FIGS. 1–3. In particular, FIG. 1 illustrates a 1,990 MHz dipole antenna from which it can be seen that the higher frequency drops off at the high end band. Moreover, as will be seen in the preferred embodiment, the gain values are insufficient. FIG. 2 also shows that a dipole antenna construction has an undesirable Voltage Standing Wave Ratio at the lower end of the frequency spectrum of interest. Finally, it can be seen in FIG. 3 that the lower frequencies of the spectrum of interest fall out of the desired matching center region. And, it has been found that such a construction does not provide the overall matching, improved electromagnetic energy transferred to and from the antenna, and desirable radiation characteristics over a wide useful range of frequencies.
In view of these shortcomings, there is a need in the art for an antenna that provides improved performance by eliminating current reversals and which does so in a small structural package while still providing all the desirable performance characteristics.